The invention relates to a colour display tube comprising in an evacuated envelope an electron gun system of the "in-line" type for generating three electron beams situated with their axes in one plane. The axis of the central beam coincides with the tube axis, and the electron beams converge on a display screen provided on a wall of the envelope. In operation the beams are deflected over the display screen in two mutually perpendicular directions by means of a first and a second deflection field. The direction of the first deflection field is parallel to the plane. The electron gun system comprises at its end curved field shapers for causing the rasters described on the display screen by the electron beams to co-incide as much as possible. Each field shaper includes at least three plates of ferromagnetic material, the plates being situated symmetrically with respect to the plane and the central beam axis, the curved field shapers facing the three beams with their concave sides and making the edge field of the first deflection field pincushion-shaped.
A frequently occurring problem in colour display tubes having an electron gun system of the "in-line" type is coma. This is expressed in the fact that the dimensions of the rasters which are written on the display screen by the three electron beams are different. This is the result of the eccentric location of the outermost electron beams relative to the field for the vertical deflection (the frame deflection field). In U.S. Pat. No. 4,196,370 a large number of patents are mentioned in which partial solutions are given. These solutions consist of using magnetic field conducting and/or screening rings and plates which are mounted at the end of the gun and which intensify or attenuate the deflection field or the deflection fields locally along a part of the paths of the electron beams. With a number of these means it is possible to cause the rasters written on the display screen by the three beams to coincide substantially. A disadvantage of the use of such means, however, is that a defocusing occurs in the outermost beams during deflection which is expressed in a distorted spot on the display screen, which spot is surrounded by a haze. One of the patents mentioned is U.S. Pat. No. 3,594,600 in which a colour display tube is described in which the rasters written by the three electron beams are made to coincide by placing two elongate C-shaped magnetic screens beyond the outermost electron beams. As a result of this the outermost electron beams are screened from the edge field of the line deflection field (the vertical field lines) while the edge field is admitted to the central electron beam. The three electron beams are screened from the edge field of the frame deflection field (the horizontal field lines) which is guided entirely around the three beams.
In Netherlands Patent Application 78 01 317 (corresponding to U.S. Pat. No. 4,237,437) laid open to public inspection a system of deflection coils is described in which field shaping means are provided in the deflection coil system. They consist, for example, of two soft-magnetic elements which are arranged diametrically opposite to each other and substantially transversely to the magnetic field of the frame deflection coil, on the neck side of the system of deflection coil, beyond the line deflection coils. A disadvantage of the use of such field-shaping means is that a great part of the frame deflection field is distorted by such means, which consume a comparatively large amount of the deflection energy.
A colour display tube of the type mentioned in the opening paragraph is described in Netherlands Patent Application 8204465 (corresponding to U.S. Pat. No. 548,276 filed 3 Nov. 1983) which has not yet been laid open to public inspection and which may be considered to be incorporated herein by reference. The field shapers described in the patent application make the first deflection field (the frame deflection field) pincushion-shaped. The pincushion-shaped field comprises substantially a two-pole field having a six-pole component. As a result of the pincushion shape the field, also for the rays of the electron beams situated not on the electron beam axes, has the correct strength and shape so that the deflection defocusing of the outermost beams is considerably reduced. In contrast with the field shapers situated in the system of deflection coils according to the above-mentioned Netherlands Patent Application 78 01 137, the field shapers are situated comparatively closely to the electron beams and only a comparatively small part of the deflection field is distorted as a result of which only little extra deflection energy is necessary.
The said Netherlands Patent Application 8204465 describes that it is useful to provide slots in the field shapers and to manufacture the field shapers from two or three plates substantially located in the elongation of each other. The object of this is to reduce the losses in the line deflection field (the second deflection field). It is also stated that by providing slots between the plates of the field shapers situated in the elongation of each other a field disturbance occurs, which will be described in detail hereinafter with reference to a figure.